Fluorinated glasses

ABSTRACT

Flourinated glasses containing indium fluoride and MF 2  fluorides in at least 70 moles %, in which M denotes one or several elements of the group Ba, Sr, Ca, Pb. Said glasses contains, in the form of stabilizing elements, either 2 to 12 % gadolinium fluoride, or 2 to 10 % magnesium fluoride, or else a mixture of both fluorides in a proportion not exceeding 20 mole %. Variants of these compositions are also described.

This application is a continuation 08/146,153 filed Jan. 24, 1994 now abandoned.

This invention concerns fluorinated glasses which can be used in optics, in particular in the infrared band, and which can be used in the manufacture of optical fibers.

Numerous studies have emphasized the interest in glasses made of heavy metal fluorides for optical transmission of up to 7 micrometers and especially the transmission by optical fibers. Further details can be found in the following specialized reference works: "Fluoride Glasses", edited by Alan Comyns, published by John Wiley & Sons in 1989, and "Fluoride Glass Fiber Optics", edited by Ishwar Aggarwal and Grant Lu, published by Academic Press in 1991.

Among the major applications of fluorinated glasses is the transmission by optical fibers in technical fields such as, for example, telecommunications, optical fiber sensors, infrared instruments and medical uses, in particular, laser surgery. In all of these applications, conventional fluorinated glasses which belong to the fluorozirconate family or fluoroaluminate family are restricted to about 4 micrometers in infrared transmission.

For this reason, attempts have been made to produce fluorinated glasses which are transparent in a wider spectral range. Standard compositions, which enable the value of the maximum transmissible wavelength to be increased, are known. Light, small and highly charged cations, for example Li⁺, Ti⁴⁺, Zr⁴⁺, Nb³⁺, Al³⁺ must be excluded. This condition is realized in several families of fluorinated glasses, such as those based on scandium, thorium, gallium and indium fluorides. In particular, Fluoroindate glasses appear promising since they display the least amount of phonon energy. Fluoroindate glasses are mentioned in FR-A-2 478 618 and EP-A-O 036 373. Numerous studies have since been carried out; reference is made, in particular, to the article "Fluoroindate Glasses" by M. Poulain, M. Poulain [sic], Y. Messadeq and A. Soufiane, published in the book "Solid State Optical Materials" in tile series "Ceramic Transactions" of the American Ceramic Society, 1992.

The simplest fluoroindate glasses are binary compositions, such as InF₃ --BaF₂. Since increasing the number of constituents is one of the conventional ways of stabilizing glasses, the compositions used as a base in manufacturing solid samples most often combine InF₃ with various divalent fluorides such as BaF₂, PbF₂, SrF₂, CdF₂, CaF₂, ZnF₂. It was also observed that yttrium fluoride stabilized InF3-BaF₂, that lanthanum fluoride can be incorporated in a limited quantity and that fluorogallate glasses were usually miscible with fluoroindate glasses. Finally, thorium fluoride, whose vitrifying properties are well known, also makes it possible to increase the stability of fluoroindate glasses and decrease the quantity of indium fluoride required to obtain proper vitrification.

Thus, fluoroindate glasses having increased stability are known and their compositions are as follows:

    ______________________________________                                         BIZYbT    30 InF.sub.3, 10 ThF.sub.4, 20 ZnF.sub.2, 30 BaF.sub.2, 10                     YbF.sub.3                                                            IZBS      40 InF.sub.3, 20 ZnF.sub.2, 20 BaF.sub.2, 20 SrF.sub.2               PZIGL     17 InF.sub.3, 19 ZnF.sub.2, 43 PbF.sub.2, 17 GaF.sub.3, 4                      LaF.sub.3                                                            ______________________________________                                    

Adjustments in the composition of the first two glasses, BIZYbT and IZBS, were made, in particular, by replacing InF₃ by GaF₃, YbF₃ by YF₃ or LuF₃, ThF₄ by ZrF₄ or HfF₄, ZnF₂ by MnF₂, BaF₂ by PbF₂, and SrF₂ by CdF₂. These experiments, which have been described in the literature, have enabled one to obtain solid samples which are approximately one centimeter thick. However, satisfactory optical fibers cannot be derived from these samples. Various laboratories have demonstrated the occurrence of superficial devitrification leading to numerous mechanical and optical defects. Moreover, certain compositions deemed to be favourable contain a considerable amount of thorium. As thorium is naturally radioactive, certain precautions are required when using it; "this is easily done in a laboratory setting but difficult to envisage in an industrial environment.

An object of the present invention is to provide compositions of Fluoroindate glasses which result in less devitrification than those fluoroindate compositions currently known.

According to one feature of the invention, it is provided that the fluorinated glass compositions contain at least 70% by weight of indium and zinc fluorides and MF₂ fluorides, where M is one or more elements of The group Ba, Sr, Ca, Pb, characterized therein that they contain in addition, as stabilizing elements, 2 to 12% gadolinium fluoride, 2 to 0% magnesium fluoride, or a mixture of these two fluorides in a proportion not exceeding 20% by weight.

According to a further feature, the fluoroindate glass is stabilized by an addition of tin fluoride and/or antimony fluoride in a proportion not exceeding 8% by weight.

According to another feature, the fluorinated glass is obtained by partially replacing indium by gallium, or zinc by manganese, in a proportion not exceeding 20% by weight. These two substitutions can be carried out independent of, or in association with, one another.

The gallium contributing to the slight increase in phonon energy must, preferably, be avoided when maximum infrared transmission is sought at increased wavelengths. Manganese can be substituted for zinc since the Fluorides of these two elements often exhibit similar vitrification levels. This substitution can then lead to a supplementary stabilization of the glass, but it can also result in a deterioration of the transmission in the near ultraviolet range and the vitreous samples then appear yellowish.

According to a further feature, the molecular composition of the Fluoroindate glass falls within the following parameters:

2 to 12% of GdF₃

25 to 45% of InF₃ +GaF₃

15 to 35% of ZnF₂ +MnF₂

25 to 45% of BaF₂ +SrF₂ +PbF₂

0 to 15% of CaF₂ +CdF₂

0 to 15% of YF₃ or yttric rare earths

0 to 12% of a fluoride belonging to the alkaline fluoride group, LaF₃ and cerium rare earths, AlF₃, ZrF₄, HfF₄ ; ThF₄.

As in experiments described in the prior art, this invention confirmed that zinc fluoride can be used to a great extent to replace indium trifluoride without significantly changing its vitreous properties. Although there is normally an optimum relationship between the relative concentrations of zinc and indium, it is the cumulative concentration of indium and zinc which appears to play a predominant role in the stability of the final glass.

Within the scope of the above-defined composition, according to the invention, it is understood that the sum of InF₃ +ZnF₂ cannot be less than 45%, that the concentration of GaF₃ and of MnF₂ cannot exceed 20% and that, in the group BaF₂ +SrF₂ +. PbF₂, one or two concentrations can be zero. Thus, the stable glasses can be free from lead or strontium. However, as already noted, increasing the number of constituents has proven to be rather flavourable to vitrification. To better understand the description, it is noted that the yttric rare earths include, in addition to yttrium, the following elements: Lu, Yb, Er, Ho, Tm, Dy and Tb, and that the cerium rare earths include: Ce, Pr, Nd and Sm.

In another embodiment of the glasses of the invention, magnesium fluoride was used as a stabilizer for the fluoroindate glasses. A particularly illustrative example is provided by the existence of ternary glasses in the system InF₃ --BaF₂ --MgF₂ which can be poured to a thickness of more than 2 mm. The ternary diagram in FIG. 1 more precisely shows the limits of the corresponding vitreous zone, line 1 defines the actual vitrification zone, and line 2 indicates the more stable glasses obtained there. In practice, however, this combination does not have sufficient stability to produce components or optical fibers. The number of constituents was thus increased to obtain a vitreous material having adequate properties.

According to another feature, the molecular composition of the Fluorinated glass falls within the following parameters:

2 to 10% of MgF₂

25 to 45% of InF₃ +GaF₃

15 to 35% of ZnF₂ +MnF₂

25 to 45% of BaF₂ +SrF₂ +PbF₂

0 to 15% of CaF₂ +CdF₂

0 to 15% of YF₃ or yttric rare earths

0 to 12% of a fluoride belonging to the alkaline fluoride group, LaF₃ and cerium rare earths, AlF₃, ZrF₄, HfF₄ ; ThF₄.

According to a further feature, the fluorinated glass contains both magnesium and gadolinium as stabilizing elements, in an overall proportion of between 2 and 20% by weight, while the remaining constituents are present in the same ratios as above.

According to another feature, the fluorinated glass contains:

2 to 6% of GdF₃

38 to 42% of InF₃ +GaF₃

15 to 25% of SrF₂ +PbF₂ +CaF₂

15 to 25% of BaF₂ +CdF₂ +LaF₃

18 to 22% of ZnF₂ +MnF₂

0 to 8% of alkaline fluoride, yttric rare earth or M'F₄ fluoride, where M'=Zr, Hf, Th.

It will be noted that magnesium, a light element, is less advantageous than indium for infrared transmission because it tends to shift the multiphonon absorption limit toward lower wavelengths. Moreover, magnesium tends to increase the resistance of the glass to corrosion due to humidity and also to increase its mechanical resistance.

The same is true for lithium which is part of the list of additives which can be incorporated to a maximum of 12% by weight. However, since this i s a monovalent or divalent cation, the contribution of these cations to the multiphonon absorption limit becomes proportionately less as the wavelength diminishes. In other words, the reduction of infrared transparency associated with the introduction of magnesium of lithium into the glass composition can be less significant between 5 and 6 micrometers than for lengths of about 8 micrometers.

These glasses are synthesized according to the conventional method for fluorinated glasses: anhydrous fluorides are mixed, fused, homogenized, poured, cooled and annealed. These .operations are conducted in such a way that contamination induced by water vapour or reactive chemical agents is negligible, whereby the tolerable contamination threshold is, of course, dependent on the intended application.

The aforementioned features of the invention will be better understood with reference to the following examples of embodiments of glasses, summarized in Tables A to C. In the attached drawings:

FIG. 1 is the ternary diagram of the InF₃ -BaF₂ -MgF₂ system mentioned

above,

FIG. 2 is the diagram of the compound system InF₃ -GaF₃ -GdF₃ of example 4 below, and

FIG. 3 is a diagram showing a spectral loss curve.

EXAMPLE 1

Using 13,45 g of In₂ O₃, 5.01 g of ZnF₂, 6.8 g of BaF₂, 6.08 g of SrF₂, 0.80 g of Gd₂ O₃ , 0.20 g of NaF and 30 g of NH₄ HF₂, a glass of the following molecular composition was prepared: 40 InF₃, 20 ZnF₂, 16 BaF₂, 20 SrF₂, 2 GdF₃, 2 NaF - Reference TO1 in Table A below. The powder mixture was prepared First, then heated for five hours at 350° C. to completely fluorinate the oxides. The crucible was then placed in a dry chamber and the temperature gradually increased to the melting point. By pouring at 650° C. into a brass mold preheated to 250° C., it was possible to obtain a parallelepiped sample measuring 60×10×10 mm after grinding and polishing.

EXAMPLE 2

According to a similar method, a series of vitreous samples were prepared having a thickness of more than 10 mm, using the same basic composition but replacing the following:

3% of NdF₃ and PrF₃ by SrF₂

2 to 12% of GaF₃ by InF₃

2 to 5% of CdF₂ by ZnF₂

1 to 8% of PbF₂ by SrF₂

This corresponds to the compositions under the references T02 to T07 in Table A.

EXAMPLE 3

Similarly, thick samples were obtained having the following molecular composition: 36 InF₃, 20 ZnF₂, 15 BaF₂, 20 SrF₂, 2 CaF₂, 2 GaF₃ PbF₂, 2GdF₃. A comparable sample was synthetized by replacing CaF₂ with the same proportion of LaF₃.

EXAMPLE 4

The system InF₃ -GaF₃ -ZnF_(b) 2 -BaF₂ -SrF₂ -GdF3-NaF was studied. By fixing the concentration at 20% of ZnF₂, 16% of BaF₂, 20% of SrF₂ and 2% of NaF, a pseudoternary InF₃, GaF₃, GdF₃ was defined and is illustrated in FIG. 2.

The zone in which glasses with increased stability are formed is shown by the area bounded by line 3. Line 4 corresponds to the range of compositions at which glasses measuring several millimeters in thickness can be obtained by more rapid cooling.

OTHER EXAMPLES

Samples of glasses based on indium fluoride stabilized by gadolinium fluoride or magnesium fluoride were synthesized to thicknesses greater than 10 mm for various compositions shown in Table B below. This Table B, in which the proportions are expressed as a percentage of molecular point T_(x), and the maximum temperature of the peak of crystallization weight, also gives the characteristic physical values, such as density, vitreous transition temperature T_(g), temperature of the crystallization under standard measuring conditions. In comparison to fluoroindate glasses described in the prior art, it is noted that incorporating gadolinium and magnesium fluorides only affects, to a limited degree, the values of the physical constants of the glass, such as density, index, dilatation, T_(g), with the exception, of course, of the crystallization temperature T_(x), which is strongly correlated to the stability of the glass.

Using the glass noted under reference T28 in Table B, it was possible to obtain optical fibers having an attenuation of less than 1 dB/m at 5.4 micrometers which only heat up slightly when they transmit a flux emitted by a CO laser. FIG. 3 shows the spectral loss curve of a fiber of this type, between 1 and 5 micrometers. It can be seen that the minimum optical loss is approximately 60 dB/km at about 2.7 micrometers.

Fibers treated with Nd³⁺ and Pr³⁺ ions show the absorption and emission characteristics expected for these rare earths within a fluoride matrix.

They are suitable for laser emission or optical amplification equipment.

Incorporating magnesium in fluoroindate glasses increases their resistance to humidity. The loss of mass of a sample containing 8% of which is subjected to a washing at 90° C. in de-ionized water is, on the average, 25% less than the loss experienced by basic glass in similar conditions. Thus, this provides a possible improvement in the behaviour of optical components made of fluoroindate glasses in humidity.

Table C shows the fluoroindate compositions stabilized by the additions of antimony fluoride and/or thorium fluoride.

                                      TABLE A                                      __________________________________________________________________________     Rel.                                                                               InF3                                                                              ZnF2                                                                               BaF2                                                                               SrF2                                                                              GdF3                                                                               GaF3                                                                               NaF MFn Density                                      __________________________________________________________________________     T01 40 20  16  20 2       2       4.99                                         T02 40 20  16  20 2       2   2 CdF2                                                                             5.03                                         T03 40 17  16  20 2       2   3 CdF2                                                                             5.05                                         T04 40 15  16  20 2       2   5 CdF2                                                                             5.09                                         T05 40 20  16  18 2       2   2 PbF2                                                                             5.07                                         T06 40 20  16  15 2       2   5 PbF2                                                                             5.15                                         T07 40 20  16  12 2       2   8 PbF2                                                                             5.37                                         __________________________________________________________________________

                                      TABLE C                                      __________________________________________________________________________     Ref.                                                                              InF3                                                                              ZnF2                                                                               BaF2                                                                               SrF2                                                                              GdF3                                                                               GaF3                                                                               M*Fn MFn  M'Fn  M"Fn Tg Index                         __________________________________________________________________________     T39                                                                               25 20  27     3            10 ThF4                                                                             10                                                                               YbF3                                                                               5 SBF3                                                                              317                                                                               1.517                         T40                                                                               25 12  22     3       13 CdF2                                                                             10 ThF4                                                                             10                                                                               YbF3                                                                               5 SbF3                                                                              302                                                                               1.526                         T41                                                                               29 20  28     2            10 ThF4                                                                             6 YF3 5 SbF3                                                                              318                                                                               1.515                         T42                                                                               25 15  28     2        5 CdF2                                                                             10 ThF4                                                                             10                                                                               YbF3                                                                               5 SbF3                                                                              318                                                                               1.522                         T43                                                                               25 20  28     2            10 ThF4                                                                             10                                                                               YF3 5 SbF3                                                                              328                                                                               1.515                         T44                                                                               25 20  22     3    5       10 ThF4                                                                             10                                                                               YF3 5 SbF3                                                                              326                                                                               1.503                         T45                                                                               35 20  23  7.5                                                                               2             5 ThF4                                                                             2.5                                                                              CdF2     308                                                                               1.507                         T46                                                                               18 16  28     3   12        9 ThF4                                                                             10                                                                               YbF3                                                                               4 MnF2                                                                              338                                                                               1.509                         __________________________________________________________________________

                                      TABLE B                                      __________________________________________________________________________     Ref.                                                                              InF3                                                                              ZnF2                                                                               BaF2                                                                               SrF2                                                                              GdF3                                                                               GaF3                                                                               NaF                                                                               MFn   M'Fn  Density                                                                             Tg Tx Tp                          __________________________________________________________________________     T08                                                                               38 20  16  20 2   2   2              4.95                                   T09                                                                               36 20  16  20 2   4   2              4.92                                   T10                                                                               34 20  16  20 2   6   2              4.91                                   T11                                                                               32 20  16  20 2   8   2              4.89 308                                                                               412                                                                               462                         T12                                                                               30 20  16  20 2   10  2              4.84                                   T13                                                                               28 20  16  20 2   12  2              4.80                                   T14                                                                               26 20  16  20 2   14  2              4.78                                   T15                                                                               20 20  16  20 2   20  2              4.71 320                                                                               430                                                                               443                         T16                                                                               10 20  16  20 2   30  2              4.61                                   T17                                                                               34 10  16  20 2   6   2  10                                                                               MnF2      4.84 294                                                                               398                                                                               425                         T18                                                                               32 20  15  15     6      12                                                                               MgF2      4.27 310                                                                               359                                                                               365                         T19                                                                               34 20  15  18 2   6      5 MgF2      4.50 311                                                                               436                                                                               447                         T20                                                                               30 20  16  20 7   5   2              5.01                                   T21                                                                               29 31  16  15 2   5   2              4.82 292                                                                               362                                                                               374                         T22                                                                               34 19  16  20 2   6   2  1 HfF4      4.92 300                                                                               420                                                                               463                         T23                                                                               34 18  16  20 2   6   2  2 CdF2      4.85                                   T24                                                                               33 18  16  20 2   6   2  1 CdF2      4.94                                   T25                                                                               33 20  16  20 2   6   2  1 PbF2      4.93                                   T26                                                                               32 20  14  14 2   6   2  10                                                                               PbF2      5.28 284                                                                               397                                                                               408                         T27                                                                               34 20  17  20 2   6      1 LaF3      4.79                                   T28                                                                               34 20  15  16 2   6   2  5 YF3       4.87 298                                                                               423                                                                               440                         T29                                                                               34 16  16  16 2   6   5  5 ThF4      5.02 301                                                                               411                                                                               420                         T30                                                                               40 20  16  17 2       2  3 NdF3           296                                                                               398                                                                               410                         T31                                                                               34 20  15  20     6      5 MgF2           296                                                                               406                                                                               444                         T33                                                                               38 20  15  20     2      3 CaF2                                                                               2 MgF2     302                                                                               388                                                                               398                         T34                                                                               34 20  15  20 2   6   2  0.5                                                                              YF3 0.5                                                                              ZrF4     298                                                                               320                                                                               460                         T35                                                                               34 20  15  20 2   6   2  0.5                                                                              CaF2                                                                               0.5                                                                              SnF2     300                                                                               384                                                                               392                         T36                                                                               34 20  15  15 5   6      5 LiF            283                                                                               414                                                                               429                         T37                                                                               30 20  16  20 7   5   2                   309                                                                               393                                                                               406                         T38                                                                               34 20  15  20     6      5 MgF2           310                                                                               367                                                                               388                         __________________________________________________________________________ 

We claim:
 1. A fluorinated glass composition comprising indium fluoride, zinc fluoride and MF₂ fluorides, wherein the sum of indium fluoride, zinc fluoride, and MF₂ is at least 70 mole % of the composition, and M is at least one element selected from a group consisting of Ba, Sr, Ca, Pb, the amount of lead fluoride being limited to 1-10 mole %, and at least one stabilizing element from a group consisting of 2-12 mole % of gadolinium fluoride, 2-10 mole % of magnesium fluoride, or a mixture of these two fluorides in a proportion not exceeding 20 mole %.
 2. The fluorinated glass composition of claim 1, wherein the fluorinated glass is further stabilized by the addition of at least one fluoride selected from the group consisting of tin fluoride, SnF₂, and antimony fluoride, SbF₃, in a proportion not exceeding 8 mole %.
 3. The fluorinated glass composition of one of claims 1 or 2, wherein the molecular composition of said glass falls within the following parameters expressed as mole %:2 to 12% of GdF₃ 25 to 45% of InF₃ +GaF₃ 15 to 35% of ZnF₂ +MnF₂ 25 to 45% of BaF₂ +SrF₂ +PbF₂ 0 to 15% of CaF₂ +CdF₂ 0 to 15% of YF₃ or rare earth fluorides wherein the rare earth is selected from group consisting of Lu, Yb, Er, Ho, Tm, Dy, and Tb 0 to 12% of a fluoride selected from the group consisting of alkali metal fluorides, LaF₃, AlF₃ ZrF₄, HfF₄, ThF₄ and fluorides of Ce, Pr, Nd, and Sm; wherein the sum of InF₂ +ZnF₂ is at least 45%, and the concentration of GaF₃ and of MnF₂ is in the range of 0-20%.
 4. The fluorinated glass composition of one of claims 1 or 2, wherein the molecular composition of said glass falls within the following parameters expressed in mole %:2 to 10% of MgF₂ 25 to 45% of InF₃ +GaF₃ 15 to 35% of ZnF₂ +MnF₂ 25 to 45% of BaF₂ +SrF₂ +PbF₂ 0 to 15% of CaF₂ +CdF₂ 0 to 15% of YF₃ or rare earth fluorides, wherein the rare earth is selected from the group consisting of Lu, Yb, Er, Ho, Tm, Dy, and Tb 0 to 12% of a fluoride selected from the group consisting of alkali metal fluorides, LaF₃, AlF₃ ZrF₄, HfF₄, ThF₄ and fluorides of Ce, Pr, Nd and Sm wherein the sum of InF₂ +ZnF₂ is at least 45%, and the concentration of GaF₃ and of MnF₂ is in the range of 0-20%.
 5. The fluorinated glass composition of one of claims 1 or 2, wherein the composition contains magnesium fluoride gadolinium fluoride as stabilizing elements in the range of 2 to 20 mole %, the remaining components of the molecular composition falling within the following parameters expressed in mole %:25 to 45% of InF₃ +GaF₃ 15 to 35% of ZnF₂ +MnF₂ 25 to 45% of BaF₂ +SrF₂ +PbF₂ 0 to 15% of CaF₂ +CdF₂ 0 to 15% of YF₃ or rare earth fluorides, wherein the rare earth is selected from the group consisting of Lu, Yb, Er, Ho, Tm, Dy, and Tb 0 to 12% of a fluoride selected from the group consisting of alkali metal fluorides, LaF₃, AlF₃ ZrF₄, HfF₄, ThF₄ and fluorides Ce, Pr, Nd and Sm; wherein the sum of InF₂ +ZnF₂ is at least 45%, and the concentration of GaF₃ and of MnF₂ is in the range of 0-20 mole%.
 6. The fluorinated glass composition of one of claims 1 or 2, wherein the molecular composition of said glass falls within the following parameters expressed in mole %:2 to 6% of GdF₃ 38 to 42% of InF₃ +GaF₃ 15 to 25% of SrF₂ +PbF₂ +CaF₂ 15 to 25% of BaF₂ +CdF₂ +LaF₃ 18 to 22% of ZnF₂ +MnF₂ 0 to 8% of alkali metal fluorides, rare earth fluorides, wherein the rare earth is selected from the group consisting of yttrium, Lu, Yb, Er, Ho, Tm, Dy, and Tb or M'F₄ fluorides, wherein M' is Zr, Hf or Th.
 7. The fluorinated glass composition according to one of claims 1 or 2, wherein indium is partially replaced by gallium, or zinc is partially replaced by manganese, in a proportion not exceeding 20 mole %, these substitutions being possible independent of one another or in association with one another. 